The present invention relates to control systems and more particularly to fluid actuators to activate process control equipment by use of pneumatic or hydraulic signals.
In the field of control systems, it has been the general practice to employ actuators to activate process control equipment in response to various types of input signals. Fluid actuators have found particular use in control systems wherein electrical signals are either not available or would be hazardous. For example, in many manufacturing processes, such as semi-conductor fabrication processes, the rate of flow of highly toxic, flammable gases must be regulated continuously throughout the process. In such cases, the use of pneumatic actuators is wide spread as a means for safely providing an activating output force for opening or closing a valve to automatically control the rate of fluid flow.
One of the most critical problems confronting developers of fluid actuators has been the reduction of their overall size. When the fluid actuator is used to control the flow of hazardous fluids, e.g., flammable or toxic gases, the fluid containers, valves, and controls are usually stored in a specially designed ventilated compartment. In order to increase the efficient use of such equipment, it is desirable to reduce the size of the valves and controls so that, for a given size compartment, the volume of space available for fluid storage is a maximum. This requirement is difficult to satisfy when it is necessary that such actuators also produce considerable output forces, as is the case in the regulation of hazardous fluids.
Prior attempts at the solution of this problem include the single piston actuator, wherein the output force is directly related to the effective area of the piston. Such actuators are not satisfactory because they usually require a considerable diameter in order to develop sufficient force. Another improvement is the multiple piston actuator. These actuators have two or more pistons in series in an attempt to increase the output force while reducing the diameter. However, such actuators are usually complex and suffer from undue height. Other actuators have used various mechanical devices, e.g., cams, rack and pinions, etc., for multiplying the output forces of a low profile device. Such attempts have often resulted in either complex, or expensive, or unacceptably large structures. In all of the above cases wherein a reasonably small actuator is designed to develop high output thrusts, short life has been a problem because of wear and fatigue in overstressed components.
For these and similar reasons, those concerned with the development of fluid actuators have recognized the need of increasing actuator output force while reducing actuator size. The present invention fulfills this need.